α-Cyclodextrin (αCD) was threaded onto 10 kDa poly(ethylene glycol) (PEG), which was then stoppered with bulky end groups (4-methoxynaphthyl or 9-anthracenylmethyl) to give polyrotaxanes containing about 38 αCD rings threaded onto a PEG backbone. The polyrotaxanes were converted into soluble macroinitiators for atom transfer radical polymerization (ATRP) by esterifying the hydroxyl groups of the threaded αCDs with 2-bromoisobutyryl bromide to a degree of substitution (DS) of 8 per αCD. Living ATRP of methyl methacrylate (MMA) from these polyrotaxane macroinitiators led to polymer brushes with molecular weights of up to 1.7 MDa. Polymer brushes were observed by atomic force microscopy. Surprisingly, large amounts of unthreaded αCD star polymer were observed by GPC. The appearance of these unthreaded αCD star polymers was attributed to the shear-induced rupture of the PEG backbone during passage of the brush through the GPC column. Backbone rupture also occurred upon heating the brushes to elevated temperatures. Proof of the bottle-brush structure was further provided without backbone rupture using diffusion ordered NMR spectroscopy.